The measurement of shear stresses in hollow axles, particular in aircraft landing gear axles, is of extreme importance in the problem, vital to flying safety, of quickly and reliably determining the weight and center of gravity of an aircraft, which of course necessitates measurement of the vertical force in each of the landing gears. According to a known proposal this force is measured directly via the oil pressure in the hydraulic shock absorbers, but it has been impossible to achieve satisfactory accuracy with this method because of the unavoidable friction of the pistons.
Another method, which in principle is more accurate and which is widely used, makes use of strain-gauge transducers which measure the shear strain of a miniature axle clamped inside the hollow axles of the landing gear by means of expanding flanges which transfer deformations of the axle to the miniature axle. To achieve accurate measuring results, therefore, the miniature axle must be absolutely securely held inside the landing gear axle, and this seems to be difficult to achieve. In practice, therefore, it has proved difficult to maintain unaltered zero point after a landing, with the considerable stresses caused in the landing gear. Especially after a hard landing the zero point may alter by several per cent which is obviously most unsatisfactory.